Final answer:
Elements and compounds are considered pure substances as they have a constant composition and set of properties. Mixtures are not pure substances since they can vary in composition and are not chemically bonded.
Step-by-step explanation:
In chemistry, pure substances are materials that have a consistent composition and set of properties throughout the sample. These include elements and compounds. An element is a pure substance that cannot be broken down into simpler substances by chemical means. Examples include gold (Au), aluminum (Al), and oxygen (O2). On the other hand, compounds are substances that can be chemically broken down into elements; they consist of two or more types of atoms bonded together. Water (H2O) is a common example of a compound.
Mixtures, however, are combinations of two or more elements and/or compounds that have not bonded chemically and can vary in composition. They can be either homogeneous or heterogeneous in nature, depending on whether the mixture is uniform throughout or not. For this reason, mixtures are not considered pure substances.
Therefore, the correct answer to the student's question is 'A and B', which means that both elements and compounds are categorized as pure substances.